In lithographic printing, a so-called printing master such as a printing plate is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a printed copy is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called “driographic” printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film (CtF) method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master. Since about 1995, the so-called ‘computer-to-plate’ (CtP) method has gained a lot of interest. This method, also called ‘direct-to-plate’, bypasses the creation of film because the digital document is transferred directly to a plate precursor by means of a so-called plate-setter.
Especially thermal plates, which are sensitive to heat or infrared light, are widely used in computer-to-plate methods because of their daylight stability. Such thermal materials may be exposed directly to heat, e.g. by means of a thermal head, but preferably comprise a compound that converts absorbed light into heat and are therefore suitable for exposure by lasers, especially infrared laser diodes. The heat, which is generated on image-wise exposure, triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer, decomposition, or particle coagulation of a thermoplastic polymer latex, and after optional processing, a lithographic image is obtained. Many thermal plate materials are based on heat-induced ablation. A problem associated with ablative plates is the generation of debris which is difficult to remove and may disturb the printing process or may contaminate the exposure optics of the plate-setter. As a result, such ablative plates require a processing step for removing the debris from the exposed material.
EP 770 497 discloses a method wherein an imaging material comprising an image-recording layer of a hydrophilic binder, a compound capable of converting light to heat and hydrophobic thermoplastic polymer particles, is image-wise exposed, thereby inducing coalescence of the polymer particles and converting the image-recording layer into an hydrophobic phase which defines the printing areas of the printing master. Subsequently the image-wise exposed precursor is developed by rinsing it with plain water or an aqueous liquid.
EP 514 145 discloses a radiation-sensitive plate which comprises a coating comprising core-shell particles having a water insoluble heat softenable core component and a shell component which is soluble or swellable in aqueous alkaline medium. The radiation causes selected particles to coalescence, at least partially, to form an image and the non-coalesced particles are then selectively removed by means of an aqueous alkaline developer.
EP 1 342 568 discloses a method wherein an imaging material comprising an image-recording layer of a hydrophilic binder, a compound capable of converting light to heat and hydrophobic thermoplastic polymer particles, is image-wise exposed, thereby inducing coalescence of the polymer particles and converting the image-recording layer into an hydrophobic phase which defines the printing areas of the printing master. Subsequently the image-wise exposed precursor is processed with a gum solution, thereby developing and gumming the plate in a single step. In this single step process, the image-recording layer at the non-exposed areas is removed with the gum solution from the support, revealing the hydrophilic surface of the support, also called “clean-out”, and simultaneously the hydrophilic surface in these non-image areas is protected from contamination (fingerprints, fats, oils, dust, oxidation, etc.) by the gum.
An important prerequisite for a high quality printing plate precursor is its shelf-life stability. A problem associated with printing plate precursors of the prior art which are based on heat-induced coagulation of thermoplastic polymer particles and are processed with a gum solution, is a lack in shelf-life stability: after ageing the precursor under high humidity condition the “clean-out” is insufficient and toning appears. “Toning” means an increased tendency of ink-acceptance at the non-image areas. It is important in the preparation of high quality printing plates to avoid toning as much as possible.